allow the deserializer to optionally handle missing fields

This allows json to deserialize missing values as a `null`.

src/bench_enum.rs

@@ -240,11 +240,6 @@ mod deserializer {
             SyntaxError
         }
 
-        #[inline]
-        fn missing_field_error(&mut self, _field: &'static str) -> Error {
-            SyntaxError
-        }
-
         #[inline]
         fn unexpected_name_error(&mut self, _token: de::Token) -> Error {
             SyntaxError
@@ -254,6 +249,13 @@ mod deserializer {
         fn conversion_error(&mut self, _token: de::Token) -> Error {
             SyntaxError
         }
+
+        #[inline]
+        fn missing_field<
+            T: de::Deserializable
+        >(&mut self, _field: &'static str) -> Result<T, Error> {
+            Err(SyntaxError)
+        }
     }
 }